1. Field of the Invention
The present invention relates to an external electrode type discharge lamp and a method of manufacturing the same, and more particularly to an external electrode type discharge lamp which is effective for preventing a glass envelope just below external electrodes from being perforated, and to a method of manufacturing the same.
2. Description of the Related Art
As for a discharge lamp, gas is encapsulated into a transparent hollow airtight envelope, and discharge is caused in the envelope, and light radiated by the discharge is taken out to the outside of the envelope, thus using the light for illumination. The discharge lamps include an internal electrode type in which electrodes are provided in an envelope causing discharge, and an external electrode type.
Among these discharge lamps, the external electrode type discharge lamp has a structural feature in that the electrodes are provided outside the envelope, and is also called a non-electrode discharge lamp because no electrodes are provided in the envelope. The external electrode type discharge lamp has the following many advantages compared to the internal electrode type discharge lamp. Since it is unnecessary to seal a lead communicating from the outside of the envelope to the inside of the envelope, the manufacture of the external electrode type discharge lamp is easy. Since the envelope can be formed to the form of a thin tube or to be thin, the external electrode type discharge lamp has the advantage in its compact size. Since the electrodes are not damaged due to electron attack during lighting-up and blurs of the inside of the discharge envelope due to sputtering of the electrode do not occur, a probability of shortening of a lifetime at the time blinking and lighting are repeated is small. Thus, the external electrode type discharge lamp is used for various applications including a light source of a backlight for liquid crystal displays and a light source for radiating light onto an original manuscript in various kinds of OA machines such as photocopying machines, facsimile apparatus, and image scanners.
Glass is mainly used for the discharge envelope, and often cylindrical. There are many sorts of shapes in the electrode, and a pair of ring-shaped electrodes encircling the outside of the cylindrical glass envelope along its circumference are arranged at intervals in a longitudinal direction of the glass envelope. This discharge envelope is called a ring type. Alternatively, there is a discharge envelope called an aperture type, in which two band-shaped electrodes extending along a longitudinal direction of the glass envelope arranged so as to be spaced from each other along a circumference of the glass envelope. Furthermore, a discharge envelope is conceived, in which parallel two elongate electrodes encircle spirally a cylindrical glass envelope (called a spiral type).
The inventors of the present invention found phenomenon that in a conventional external electrode type discharge lamp, a hole begins to open at a portion just below an external electrode of a glass envelope from within the glass envelope with the course of lighting-up time, and the hole becomes large so that the edge of the hole reaches the external surface of the glass envelope, leading to incapability of keeping air-tight of the glass envelope. This “hole-perforation” phenomenon is particularly remarkable when mercury gas is contained in gas as discharge medium, and the inventors of the present invention confirmed that this phenomenon appears at an early stage, and that also a rate of progress of the phenomenon is fast.
It is proved that this phenomenon can be reduced by lowering a high frequency AC voltage to be applied between the external electrodes. However, although a lifetime characteristic is slightly improved by lowering the voltage, another problem occurs that a brightness of a lamp decreases by just that much.